Transcript ppt - Courses

Physical Layer
IS250
Spring 2010
[email protected]
Summary
 Physical layer is concerned with the communication of
data encoded as signals transmitted over a medium
- Fundamental techniques: encoding, modulation, multiplexing
 Channel capacity influenced by hardware bandwidth,
encoding scheme, transmission impairments (noise and
attenuation)
2
Outline
 Fundamental concepts
-
Data, signal, transmission (Ch. 5)
Transmission media (Ch. 7)
Multiplexing (Ch. 11)
Transmission impairments (Ch. 8.2)
 Data encoding (Ch. 6, 10)
 Channel capacity (Ch. 7)
3
Communication System
 Transmitter, receiver, medium
http://i.ehow.com/images/GlobalPhoto/Articles/4996474/illustration-main_Full.jpg
4
Communication System
 Transmitter, receiver, medium
 Data, Signal, Transmission
- Data: entities that convey meaning (can be digital or
analog)
- Signals: electric or electromagnetic representations of
data (can be digital or analog)
- Transmission: communication of data by propagation
and processing of signals
5
Data and Signal

Digital data, digital signal

Analog data, digital signal
Data

Digital data, analog signal

Analog data, analog signal
6
Transmission Media
 Guided (wired): twisted pair, coaxial cable, optical fiber
 Unguided (wireless): RF, microwave (terrestrial & satellite), infra-red
7
Frequencies you may be
using today





Radio: 535-1605kHz (AM); 88-108MHz (FM)
TV: 54-88MHz; 174-216MHz; 470-806MHz
Cell phones: 850, 900, 1800, 1900MHz
Cordless phones: 900MHz, 2.4GHz, 5.8GHz
Wi-Fi: 2.4GHz (802.11b/g); 5GHz (802.11a)
 Q: how do radio/tv stations and receivers, cell phones
and towers, etc., share the airwaves?
 Q: how are 500 channels of TV programming sent over
the cable?
8
Multiplexing
 Combining multiple data streams into a single signal
- Allows resource sharing (e.g., of a communication channel)
 Many different forms of multiplexing
- Time division multiplexing (TDM)
- GSM, SONET
- Frequency division multiplexing (FDM)
- Applications: Broadcast radio/TV, DSL
- Wave division multiplexing (WDM) for fiber optic communication
- Orthogonal FDM (OFDM) used in DSL, 802.11, 802.16, etc.
- Spread spectrum
- Flavors: Frequency hopping (FHSS), direct sequence (DSSS)
- Transmitter & receiver coordinates via pseudo-random number
generator
- Basis for CDMA (code-division multiple access) technologies
- Spatial multiplexing
- e.g., wireless MIMO antennae used in 802.11n
9
Outline
 Fundamental concepts
-
Data, signal, transmission (Ch. 5)
Transmission media (Ch. 7)
Multiplexing (Ch. 11)
Transmission impairments (Ch. 8.2)
 Data encoding (Ch. 6, 10)
 Channel capacity (Ch. 7)
10
Transmission Impairments
 Signal received may differ from signal
transmitted
- Analog transmission: degradation of signal quality
- Digital transmission: bit errors
 Causes
- Attenuation
- Noise
Source: http://www.telebyteusa.com/primer/fig9.gif
11
Attenuation and Noise
 Attenuation
- Signal strength falls off with distance
- Received signal strength:
- must be enough to be detected
- must be sufficiently higher than noise to be received without error
- Attenuation is an increasing function of frequency
 Noise: additional signals inserted between transmitter
and receiver
- Thermal: thermal agitation of electrons (also called “white noise”)
- Intermodulation: signals that are the sum and difference of original
frequencies sharing a medium
- Crosstalk: signal from one line is picked up by another
- Impulse: irregular pulses or spikes that are high in amplitude and short
in duration, e.g., external electromagnetic interference
12
Analog v. Digital Transmission
 Digital transmission better than analog transmission in
supporting long distance communication. Why?
 Analog signal transmitted without regard to content
- Signal is subject to attenuation and noise
- Amplifiers can be used to boost signal strength, but noise is
also amplified
 Digital transmission involves processing of content
- Signal is subject to attenuation and noise
- Repeaters can be used to boost signal strength
- Repeater receives signal, extracts bit pattern, retransmits clean
signal without noise
- Attenuation is overcome, and noise is not amplified
13
Outline
 Fundamental concepts
-
Data, signal, transmission (Ch. 5)
Transmission media (Ch. 7)
Multiplexing (Ch. 11)
Transmission impairments (Ch. 8.2)
 Data encoding (Ch. 6, 10)
 Channel capacity (Ch. 7)
14
Encoding Techniques

Digital data, digital signal

Analog data, digital signal
Data

Digital data, analog signal

Analog data, analog signal
15
1. Digital Data, Digital Signal
 Digital signal as discrete, discontinuous voltage pulses
- Binary data encoded into signal elements
- Bit duration (function of data rate), voltage levels have to be specified
 Example 1: RS-232
 Example 2: USB
- USB uses NRZI (non-return-to-zero inverted) encoding
- Presence of transition encodes a “1”
- Absence of transition encodes a “0”
- Data rates: 1.5Mbps, 12Mbps, 480Mbps
3.2v
0v
16
2. Analog Data, Digital Signal
 Step 1: convert analog data
into digital data via sampling
and quantization (e.g., pulse
code modulation)
- Example: 4-bit PCM
-
Analog data input (in red)
16 quantized levels can be represented using 4 bits
Therefore each sample converted into 4 binary bits
Digital data output: 1001101111001101111011101111…
 Step 2: digital data can then be transmitted using
digital encoding schemes (previous slide)
 Variations: delta PCM, adaptive DPCM
17
3. Analog Data, Analog Signals
 Example: broadcast radio, TV
 Carrier signal modulated by analog
data
 Types of analog modulation
carrier
data
- Amplitude modulation (AM)
- Frequency modulation (FM)
- Phase modulation (PM)
 Why modulate analog signals?
- Higher frequency can give more
efficient transmission
- Permits frequency division
multiplexing by using different carrier
frequencies for different channels
(see slide on multiplexing)
18
4. Digital Data, Analog Signal

Example: using a modem (modulatordemodulator) to send data over analog
public telephone system

Digital Modulation very similar to Analog
Modulation:
-
-

ASK (amplitude shift keying): values
represented by different amplitudes of
carrier
- Usually, one amplitude is zero, i.e., detect
presence or absence of carrier
FSK (frequency shift keying): values
represented by different frequencies (near
carrier)
PSK (phase shift keying): phase of carrier
signal shifted to represent data
Can be combined: e.g., QAM (quadrature
amplitude modulation) is combination of
ASK and PSK
19
Outline
 Fundamental concepts
-
Data, signal, transmission (Ch. 5)
Transmission media (Ch. 7)
Multiplexing (Ch. 11)
Transmission impairments (Ch. 8.2)
 Data encoding (Ch. 6, 10)
 Channel capacity (Ch. 7)
20
Channel Capacity
 Hardware cannot change signal states
(e.g., voltage levels) instantaneously
 transmission systems have limited bandwidth
 Bandwidth (B): maximum rate that the hardware can
change a signal (measured in Hertz, or cycles per
second)
 Data rate (D): rate at which data can be communicated
(measured in bits per second)
 Channel capacity (C): maximum data rate, which is
determined by hardware bandwidth
21
Channel Capacity
 Nyquist (1928): D < 2B
- the number of independent pulses that could
be put through a telegraph channel per unit
time is limited to twice the bandwidth of the
channel
 Hartley (1928): D < 2B log2(K)
- where K is the number of distinct messages
that can be sent
- Nyquist result is special case of K=2
22
Example
dial-up modem w QAM (Comer 10)
 B = 2400Hz
 V.32 modem:
- K = 32
- D < 2*2400*log232 = 24000bps
 V.32bis modem:
- K = 128
- D < 2*2400*log2128 = 33600bps
 But these modems can only
support data rates of 9600bps
and 14400bps, respectively.
Why?
23
Shannon’s Theorem (1948)
 Channel capacity in the presence of noise:
C = B log2(1+S/N)
Where
- C is effective channel capacity
- B is hardware bandwidth
- S/N is the Signal-to-Noise Ratio
24
Decibels (dB)
 Engineers like to express signal-to-noise ratio in
decibels (dB) using the following quantity:
10log10(S/N)
 Example: a signal-to-noise ratio of 100 is
expressed as 20dB
 Example: a signal-to-noise ratio of 30dB is the
same as 10^(30/10) or 1000
25
Application
 Conventional telephone system
-
Engineered for voice
Bandwidth is 3000Hz
SNR ~= 30dB
Effective capacity is:
3000log2(1+1000) ~= 30000bps
- Conclusion (Comer, p.130): dial-up modems have little
hope of exceeding 28.8Kbps
- Q: So what about those 56k modems?
26
Implications
 Nyquist/Hartley: encoding more bits per
cycle will improve data rate
 Shannon: no amount of clever
engineering can overcome the
fundamental physical limits of a real
transmission system
27
Summary
 Physical layer is concerned with the communication of
data encoded as signals transmitted over a medium
- Fundamental techniques: encoding, modulation, multiplexing
 Channel capacity influenced by hardware bandwidth,
encoding scheme, transmission impairments (noise and
attenuation)
28